Fabrication of thin film transistors and other thin film devices entails the deposition of a number of thin layers upon one another and their patterning by various standard methods. Amorphous silicon thin film devices are commonly made by the PECVD (plasma enhanced chemical vapor deposition) method because it is a low temperature process compatible with these materials. Simply stated, PECVD employs an RF plasma for decomposing gases whose constituents chemically react and deposit upon a substrate. In accordance with this method, it is possible to deposit layers of different materials, one upon the other, by simply changing the reactant gases. Then the substrate is removed from the plasma chamber for patterning the exposed surface layer, which is most commonly accomplished by coating it with a photoresist layer, photolithographically exposing the photoresist, removing selected areas of the photoresist to expose the underlying layer, wet etching the layer, and finally removing the remaining photoresist.
When making thin film amorphous silicon electronic devices it is often necessary to deposit a layer of n+ amorphous silicon. Generally, no problems are encountered when depositing the n+layer as one of a series of layers, in an evacuated chamber. However, we have observed that a serious defect, in the form of delamination bubbles, arises when the substrate has been exposed to air immediately prior to the n+deposition step(see the photograph of FIG. 1). In that 170X photograph there can be seen stripes of amorphous silicon 10 (deposited upon a silicon nitride layer) with an overlying layer of n+ morphous silicon 12. Randomly appearing circles 14 throughout the photo represent delamination bubbles of various sizes. This defect causes major problems, namely, the lack of good ohmic contact between the n+ and its underlying layer, and the disintegration of the n+ domes in subsequent processing, such as by being crushed during contact printing of a lithographic mask, so as to generate opens through the n+ layer and a profusion of contaminating particles.
It is the primary object of this invention to provide an inexpensive fabrication technique which will eliminate the formation of delamination bubbles in the n+ amorphous silicon layer, but which will not introduce any undesirable constraints on the existing thin film fabrication process.
It is another object of this invention to improve the interface bonding between the n+ morphous silicon layer and its contaminated underlying layer.